Because you have a top-N filter, that's a table calc, which means that ALL the rows are still in the underlying table. (And that's good for you.) So a WINDOW_AVG of the SUM([whatever]) will give you an average of everything. (That's one calc. Make that a reference line.) And a WINDOW_AVG of :
IF ATTR([Practice]) = (the target practice) THEN SUM([whatever]) END
will get you the average for the practice.
If you need help doing this, you'll have to upload a sample workbook.